The present invention pertains to inkjet ink and more particularly to an inkjet ink comprising self dispersing pigment and a certain aqueous vehicle which provides long latency.
inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
An ink-jet ink is characterized by a number of necessary properties, including color, jettability, decap time (latency), drying time and shelf-life, among others. There is, however, often a tradeoff between these properties because improving one property can result in the deterioration of another property.
The decap time of the ink is the amount of time a printhead can be left uncapped and idle and still fire a drop properly—that is to say without misdirection, loss of color or unacceptable decrease of velocity. Decap is sometimes referred to in the art as “latency” and these two terms will be used interchangeably.
Because not all the nozzles of the printhead print all the time, a printer service routine requires the idle nozzles to discharge (“spit”) on a regular basis into the waste container (“spittoon”) to avoid printing defects. It is desirable, however, to service the printhead as infrequently as possible as it is wasteful of ink and slows print speeds. To reduce need for servicing, an ink will preferably have a long decap time.
Contributing to decap problems is the trend for printheads to fire smaller drops to increase image resolution. The increased surface area to volume to the smaller drops allows faster evaporation of volatile vehicle components at the nozzle face and thereby tends to decrease decap time.
Both dyes and pigments have been used as colorants for inkjet inks and both have certain advantages. Pigment inks are advantageous because they tend to provide more water-fast and light-fast images than dye inks. Also, with regard to black inks, carbon black pigment can provide much higher optical density than any available dye colorant.
Pigments, in order to be used in inks, must be stabilized to dispersion in the ink vehicle. Stabilization of the pigment can be accomplished by use of separate dispersing agents, such as polymeric dispersants or surfactants. Alternatively, a pigment surface can be modified to chemically attach dispersibility-imparting groups and thereby form a so-called “self-dispersible” or “self-dispersing” pigment (hereafter “SDP(s)”) which is stable to dispersion without separate dispersant.
SDPs are often advantageous over traditional dispersant-stabilized pigments from the standpoint of greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink.
Various inkjet ink formulations comprising SDP have been disclosed in the art. For example, U.S. Pat. No. 6,153,001 discloses an aqueous SDP ink comprising diethylene glycol and urea and an aqueous SDP ink comprising propylene glycol and ethylene urea. U.S. Patent Publication 2005/0098063 discloses aqueous SDP inks comprising trimethylene urea or 1,3-dimethyl urea humectant. U.S. Pat. No. 6,069,190 discloses an aqueous inkjet ink with improved latency comprising 2-pyrrolidone humectant and a combination of SDP and anionic dye colorant. U.S. Pat. No. 6,786,957 discloses aqueous inkjet inks comprising colorant, which can be a dye, pigment or macromolecular chromophore, and an aprotic polar solvent which can be tetramethyl urea.
Although current SDP ink compositions are being successfully jetted, there is still a need in the art for, and it is an object of this invention to provide, inks with longer decap times that still retain other beneficial print properties.